Syrian army troops backed by war planes advanced to within several kilometers of Palmyra on Thursday, battling Islamic State group fighters outside the famed ancient city, a monitor said.

The Syrian Observatory for Human Rights said government troops were now some five kilometers (three miles) west of the city and engaged in fierce clashes with forces from the extremist group.

At least 19 civilians, including five children, were killed by a Syrian regime barrel bomb attack and rebel rocket fire in Aleppo, a monitoring group said on Thursday.

Fifteen of the civilians, among them four of the children and a pregnant woman, were killed in a barrel bomb strike on Wednesday evening on a rebel-held district of the divided northern city, the Syrian Observatory for Human Rights said.

More than four million Syrians have fled the civil war ravaging their country to become refugees in the surrounding region -- a million of them in the past 10 months alone, the United Nations said Thursday.

"This is the biggest refugee population from a single conflict in a generation," UN refugee chief Antonio Guterres said in a statement.

Kurdish forces backed by U.S.-led strikes hunted down Islamic State group jihadists Wednesday after fending off an assault on a town near IS' de facto Syrian capital, the Kurds and a monitor said.

Kurdish forces had seized Ain Issa on June 23, but on Monday jihadists launched a new assault on the town located 55 kilometers (35 miles) from Raqa.

The Belgian government said Wednesday that it has in recent months helped 244 Christians to escape from Syria and move to Belgium with refugee status, most of them families with children.

The humanitarian operation began in May to "save" some of the most vulnerable people in Aleppo, Syria's second city where Islamist rebels are gaining ground, Foreign Minister Didier Reynders told a press conference in Brussels.

A senior U.S. delegation ended two days of talks Wednesday with Turkish officials on escalating the fight against Islamic State (IS) jihadists in Syria, sources said.  

The delegation, including U.S. presidential envoy John Allen and Under Secretary of Defense for Policy Christine Wormuth, met in Ankara with Turkish military officials and foreign ministry undersecretary Feridun Sinirlioglu, they said.

Hackers claiming to be affiliated with the Islamic State group took down the website of the Syrian Observatory for Human Rights monitor on Wednesday and threatened its director.

The group calling itself the Cyber Army of the Khilafah (Caliphate) replaced the front page of the war monitor's site with a photoshopped image of the Observatory's director and text threatening him.

Turkey's disaster management agency is readying a giant new refugee camp to house 55,000 people in the south of the country in anticipation of a new wave of migrants fleeing the civil war in Syria, reports said Wednesday.

The head of Turkey's disaster management agency, Fuad Oktay, was quoted by local media as saying that as many as 100,000 more refugees could arrive in a 24-hour time span, given the increasingly fragile security situation in Syria.

Only about 60 Syrian rebels are being trained by the United States to take on the Islamic State group, Defense Secretary Ashton Carter said Tuesday, admitting it was far below the number hoped for.

The disclosure is likely to add to criticism of the Obama administration's military strategy, with U.S. Senator John McCain saying that the United States was "losing" the fight against the extremists, who have overrun large areas of Syria and Iraq.

U.S. Senator John McCain blasted the military strategy against the Islamic State group Tuesday, suggesting the United States was "losing" the fight and criticizing the pace of training for Syrian rebels.

Several lawmakers including McCain, the 2008 Republican presidential nominee, grilled Defense Secretary Ashton Carter and the military's top general, Martin Dempsey, on topics ranging from Ukraine to the Middle East.